A tower for a wind turbine contains typically two elements. The first tower element is the foundation, while the second tower element is the tower itself.
There are numerous ways to design the foundation. Typically the foundation is built up by help of reinforced concrete. The foundation is subdivided, thus it covers a spread foundation-area. This type of foundation is very suitable for sites with a bearing soil.
The tower is exposed to dynamic loads and to static loads. The dynamic loads are caused by the wind and are caused by the supported wind turbine, which is mounted on top of the tower.
Wind turbine towers are made of concrete. Especially for offshore sites the towers are constructed in a way that a long life time of 20 years or more is ensured.
The concrete of an offshore-tower has a dampening property, which is higher than the damping property of other materials.
Preferably pre-stressed concrete is used for towers, thus a high fatigue resistance is achieved. Thus the risk of dynamic failure is avoided.
Pre-stressed concrete towers show good fatigue properties in comparison with steel. Additionally the pre-stressed concrete tower is cheaper than others due to the lower material costs.
A typical concrete tower is known from the document WO 2009 056 898 A1 for example.
A preferred wind turbine tower is conical. The tower contains a number of segments, which are pre-stressed or which are post-tensioned.
Preferably each tower-segment contains a number of tendons, which are used to ensure and to connect it with other segments. Thus the tendons are secured at various heights of the resulting tower. They are anchored at or near the foundation of the tower.
FIG. 3 shows a well known structure of a concrete tower T. The tower contains three segments SEG1, SEG2 and SEG3.
The top segment SEG3 is secured by 24 tendons TD for example. The 24 tendons TD are connected with the resulting tower T near the top of the tower T. They are also connected with the resulting tower T in the area, where segment SEG2 and segment SEG1 are joined together. Last they are connected with the resulting tower T near or at the foundation of the tower T.
The segment SEG2 is secured by 6 additional tendons TD for example. The 6 additional tendons TD are connected with the resulting tower T near the area, where segment SEG2 and segment SEG1 are joined together. They are connected with the resulting tower T near or at the foundation of the tower T.
Thus the top view of the tower T shows channels for 24 tendons TD within the cross section of SEG3, while the top view of the tower T shows channels for 30 tendons TD within the cross section of SEG1.
To secure and to anchor all the tendons in the different cross sections is complex and time-consuming. Thus this work is a serious point within the calculation of the tower.
The most tension forces acts on the lower segments as the tendons are anchored at various heights.
Thus the lower segments have a larger cross sectional area of concrete than the upper segments.
The top segment shows a smallest cross sectional area of concrete, thus the smallest tension is applied there.